1. Field of the Invention
The present invention relates to a piezoelectric driven MEMS (Micro-electro-mechanical System) actuator.
2. Related Art
Recently, actuators fabricated by using the MEMS technique are drawing the attention. The actuators are driven by electrostatic force, thermal stress, electromagnetic force, piezoelectric force, and so on, and bent and displaced. An actuator using a piezoelectric thin film to rotate a shaft attached to a free end is known (see, for example, JP-A 2003-181800 (KOKAI)).
Furthermore, MEMS devices such as variable capacitance type capacitors and switches using actuators are being studied. The variable capacitance type capacitors and switches using the MEMS technique has, for example, a configuration in which an actuator supported at its first end by a strut provided on a substrate is provided, a movable electrode serving as an action part is provided at a second end of the actuator, a fixed electrode is provided on the substrate surface so as to be opposed to the second end of the actuator, and the distance between the movable electrode and the fixed electrode is changed by the actuator. In other words, the actuator serves as a movable beam.
Especially in the variable capacitance type capacitor including a piezoelectric driven actuator which uses an inverse piezoelectric effect or an electrostrictive effect as drive force for the movable beam, the spacing between the movable electrode and the fixed electrode can be changed sharply and continuously, and consequently the capacitance change rate can be made large. Furthermore, since air or gas can be used as a dielectric between the movable electrode and the fixed electrode, a very large Q factor is obtained. Thus, the variable capacitance type capacitor has a large number of advantages.
Furthermore, it is also possible to cause the variable capacitance type capacitor structure to function as a switch by bringing the movable electrode into contact with the fixed electrode via an extremely thin dielectric film (capacitive type) or bringing the movable electrode into contact with the fixed electrode directly (contact type). Such a switch fabricated by using the MEMS technique has both low on-resistance and high off-condition isolation characteristics as compared with the semiconductor switch, and it is also drawing keen attention.
However, the piezoelectric driven actuator is supported in the air, and has a long thin beam structure including a piezoelectric film interposed between upper and lower electrodes. Therefore, there is a very serious problem that the beam is warped upward or downward by slight residual stress in the material of the piezoelectric film. As a result, it is very difficult to adjust the capacitance value of the variable capacitance type capacitor obtained before and after applying the voltage in conformity with the design and make the drive voltage of the switch a constant value.
For example, in the action part connected to the piezoelectric driven actuator, a bendable displacement quantity D caused by the inverse piezoelectric effect is represented by the following expression.D˜E·d31·L2/t   (1)Here, E is an electric field applied to the piezoelectric film, d31 is a piezoelectric strain constant, L is the length of the actuator, and t is the thickness.
On the other hand, denoting residual strain by Sr, warp Dw of the piezoelectric driven actuator caused by residual strain which is generated in the piezoelectric film when forming the film is approximated by the following expression.DW˜Sr·L2/t   (2)
As appreciated by comparing the expression (2) with the expression (1), the displacement quantity D and the warp DW have similar relations to the length L and the thickness t of the piezoelectric driven actuator. The displacement quantity D and the warp DW are in proportion to the square of the length L, and are inverse proportion to the thickness t. For example, if the length L of the actuator is increased or the thickness t is decreased in order to widen the drive range of the piezoelectric driven actuator, the quantity of the warp DW also increases accordingly. In making the piezoelectric driven range D greater than the warp DW, therefore, geometric contrivance concerning the actuator brings about little effect. There are no ways other than making the absolute value of the residual strain Sr small as compared with the absolute value of piezoelectric strain (E·d31) caused by the inverse piezoelectric effect.
For obtaining a high quality film as regards lead zirconate titanate (PZT) known as a piezoelectric film having a great inverse piezoelectric effect, it is necessary to form a film at the room temperature and then conduct annealing at approximately 600° C. Since the volume contraction is caused by the annealing, the residual strain of the piezoelectric film formed of PZT increases inevitably. On the other hand, aluminum nitride (AlN) and zinc oxide (ZnO) used as the material of the piezoelectric film, which can be formed near at the room temperature and can be controlled in film forming residual stress comparatively precisely by using the film forming condition, are smaller in inverse piezoelectric effect by at least one digit than PZT.
If a piezoelectric material that is great in inverse piezoelectric effect is used as the piezoelectric film of the piezoelectric driven actuator in order to increase the piezoelectric strain, it becomes difficult to control the residual strain and the warp cannot be controlled, as described above. If a piezoelectric material that is comparatively easy in control of residual strain in the piezoelectric film is used, then the inverse piezoelectric effect is small and the piezoelectric driven range cannot be made sufficiently large as compared with the warp of the actuator.
Industrial application of the piezoelectric driven actuator is obstructed by such problems. Because of a great problem in the structure of the piezoelectric driven actuator, i.e., its structure which is thin in thickness and long, the piezoelectric driven actuator is warped largely by slight residual stress. Therefore, it is difficult to fabricate variable capacitance type capacitors so as to yield a constant capacitance or keep drive voltages of switches constant.